Unfortunately, there is no such thing as a hydrogen well. the only way to obtain the stuff is to break existing hydrogen-containing molecules, a process which requires an energy input.
Electrolyzing water requires an energy input equal to the energy available from burning the hydrogen back into water. In fact, because no reversible process is ever 100% efficient in this vale of tears, electrolysis actually runs at a net energy loss.
So, where does the energy needed to produce hydrogen come from ? Fossil fuels? Nuclear power?
No, hydrogen cars aren't likely to seriously impact the oil companies; the biggest advantage they have is that they permit us more control over how and where the pollution incident to our energy use is emitted. Since it's easier to implement and oversee pollution control technology in a small number of large power plants than in millions of individual vehicles, hydrogen cars could well make the air of our cities much more breathable.